Torsion oscillators are known, although not widely employed. U.S. Pat. Nos. 4,762,994 to Byerly et al., 5,543,956 to Nakagawa et al. and 5,767,666 to Asada et al. are illustrative. An illustration of a galvanometric torsion oscillator is shown in FIG. 1. (The term galvanometric is believed to be a reference to coils on the turning member operated in the manner of a common galvanometer.)
The torsion oscillator of FIG. 1 comprises a central rectangular plate 1 suspended by two extensions 3a, 3b of the material of plate 1. Extensions, 3a, 3b are integral with a surrounding frame 5. Typically, the plate 1, extensions 3a, 3b and frame 5 are cut or etched from a single silicon wafer. A coil 7 of conductive wire and a region 9 of reflective mirror material are placed on the central plate.
This entire assembly is located inside a uniform magnetic field 11 (shown illustratively by lines with arrows), such as from opposing permanent magnets (not shown). When a current passes through coil 7, a force is exerted on coil 7 which is translated to plate 1 since coil 7 is attached to plate 1. This force causes rotation of plate 1 around extensions 3a, 3b which twist with reverse inherent torsion.
Other means may be employed to make such a system oscillate, such as static electricity or extermal magnetic fields. Various ones of such means are known in the prior art. The use of a coil drive by electric current in the embodiments disclosed herein should be considered illustrative and not limiting.
The spring rate of extensions 3a, 3b and the mass of plate 1 constitute a rotational spring-ass system with a specific resonant frequency. Plate 1 can be excited to oscillate at the resonant frequency with an alternating level passing through the coil and having a frequency at the resonate frequency or having some other frequency, such as harmonic at the resonate frequency. Where the input frequency varies from the resonant frequency and is substantial in power, plate 1 oscillates at the input frequency but drive level to coil 7 must be higher to achieve the same sweep (extent of oscillation) of plate 1. The device functions as a laser scanner when a laser is directed at the oscillating surface of mirror 9, thereby replacing the much bulkier rotating polygonal mirror widely used in laser printers and copiers. Torsion oscillators also have other applications, such as to drive a clocking device, in which mirror 9 would not be used.
The angle of mirror 9 moves sinusoidally with respect to time at a certain amount of sweep (termed amplitude), in a certain repetition rate (termed frequency), and with a potential lack of symmetry with respect to the using apparatus (termed median offset). These elements must be stabilized for useful operation. But the characteristics of a torsion oscillator can vary significantly from manufacturing tolerances and changing environmental conditions. Moreover, the direction of frequency drift is not readily determined since amplitude falls for drift to both higher and lower frequency. This invention provides two alternative control procedures which stabilize operation as the resonant frequency shifts during use.